It is known that biphenyl compounds can be produced by the reductive coupling of aryl halides. For example, Chao, et al., in the Journal of Organic Chemistry, Volume 48, at pages 4904-4907 (1983), disclose the reaction of aryl halides with an equivalent amount of a highly activated metal such as nickel powder.
An alternative approach is to activate an aryl halide by a chemical transformation, and then allow the activated aryl halides to couple to form biaryl species. For example, Gilman, et al., in the Journal of the American Chemical Society, Volume 61, at pages 957-959 (1939), demonstrated this approach by the reaction of two equivalents of aryl Grignard reagents with one equivalent of nickel(II) salts to give biaryl compounds. This reaction is believed to proceed through the bis(aryl)nickel(0) species which then decomposes to give the desired biaryl product.
Kumada, et al., in Bulletin of the Chemical Society of Japen, Volume 49, at pages 1958-1969 (1976), have demonstrated that aryl halides can be reacted with a variety of aliphatic Grignard reagents to give alka-aryl products. However, attempts to couple aryl Grignard reagents with aryl halides were successful only with aryl bromides. Attempts to use aryl chlorides gave less than a ten percent yield of desired biaryl products.
Aryl chlorides are frequently more readily available than are the corresponding bromides and iodides. The chlorides are also typically less reactive and are less expensive materials as well. It would, therefore, be desirable to find a means to promote the coupling of aryl chlorides to produce high yields of biaryl compounds.
Such coupling of aryl chlorides has been disclosed by Colon, et al., in U.S. Pat. No. 4,263,466. The authors disclose the use of a metallic reducing agent such as zinc, magnesium, or manganese in a dipolar, aprotic solvent such as dimethylformamide with a catalyst containing a nickel compound in combination with triaryl organophosphines and alkali metal halide promoters. The reducing metal converts the nickel salts into highly reactive zerovalent nickel compounds which promote the coupling of the aryl halides and regenerate the nickel salts which can be reduced again to the zerovalent state, thereby maintaining the catalytic cycle.
In later publications by the same authors (see Colon, et al., in Journal of Organic Chemistry, Vol. 51 at pages 2627-2637 (1986), it is indicated that bidentate ligands (as opposed to the monodentate triaryl organophosphines disclosed in '466) are not effective for the nickel/reducing metal-promoted reductive coupling reaction. Takagi, et al., in the Bulletin of the Chemical Society of Japan, at pages 1887-1890 (1984) have made similar observations. See especially Run Number 14 reported at page 1888.